What is causing our reactive nitrogen problem?

Nutrient pollution is emerging as a big issue for many areas of the United States, and nitrogen is a major part of the problem.

Here’s a quick refresher on the natural nitrogen cycle. Plants need nitrogen to grow and flourish. Nitrogen exists naturally in our atmosphere and is actually very abundant—almost 80% of the air we breathe is made up of nitrogen—but that nitrogen is in the form of inert gas that plants can’t use. It has to be converted, or “fixed,” first. That job usually is done by a specialized type of bacteria found in the soil. After plants die, other types of bacteria eventually return the fixed nitrogen back to the atmosphere as inert gas.

At least, that’s how it worked until the beginning of the 20th century, when new synthetic nitrogen-based fertilizer first became widely available for commercial use. Farmers applied it liberally to their fields, realizing bigger plants and higher crop yields as a result of the higher amount of nitrogen available to the plants.

Although farmers, the fertilizer industry, and government agencies have improved the efficiency of nitrogen use in farm fields over the past 30 years, nitrogen loss to the environment from agriculture still remains a significant problem. Applied nitrogen fertilizer not used by the plants can wash into and degrade surrounding waters in the form of nitrate. Nitrate overload has been linked to both drinking water contamination and hypoxia—oxygen levels that are too low to support animal life—in aquatic ecosystems, sparking serious concerns among the communities that rely on that water for drinking, fishing, and recreation.

According to Daniel Sobota, a National Research Council Fellow who has spent the last 3 years studying nitrogen sources in the U.S., efforts to address these concerns first require accurate estimates of how much reactive nitrogen exists in our environment, where we add to it, and where it all winds up.

The uncertainties surrounding nitrogen source information prompted Sobota to synthesize several decades of research on the subject, resulting in a series of spatial distribution maps that illustrate both human-made nitrogen sources and sources that occur naturally in the environment.

U.S. Environmental Protection Agency (EPA) Ecologist Jana Compton collaborated with Sobota on the study. “Getting a handle on sources of nitrogen and impacts of nitrogen on ecosystems services and human health and well-being is a big focus of EPA’s Sustainable and Healthy Communities Research Program,” says Compton. The researchers estimate that in the U.S., the annual nitrogen input to the environment is, on average, three times what it was prior to European settlement. That annual input measures upwards of 25 teragrams—that’s 25,000,000 metric tons, roughly the mass of 75 Empire State Buildings.

Furthermore, they found that nitrogen isn’t distributed evenly across the landscape—although humans have increased nitrogen inputs virtually everywhere in the continental U.S., some areas remain virtually undisturbed, whereas others receive around 35 times the amount of nitrogen that exists naturally in the environment.

The Midwest, Mid-Atlantic, Central California, and parts of the Columbia River Valley in Washington and Oregon are the regions that currently see the highest nitrogen inputs.

“It’s pretty clear that synthetic fertilizer is the largest source of nitrogen to the U.S. landscape currently,” Sobota says. “But we’ve also found that other anthropogenic sources, such as biological nitrogen fixation by soybeans and alfalfa or livestock manure, can contribute a large portion of nitrogen to different parts of the country.”

Atmospheric deposition of nitrogen, caused in part by burning fossil fuels, is shown to be another big contributor, particularly along the Eastern Seaboard.

Compton believes that the research will be invaluable to EPA’s future efforts to help states develop nutrient criteria for rivers, lakes, and other bodies of water and to identify watersheds to prioritize in nutrient reduction efforts. “Watersheds that carry higher nitrogen loads would have important consideration in developing these priorities,” she explains.

The information presented in the spatial distribution maps also will be incorporated into EnviroAtlas, EPA’s interactive, Web-based application for viewing and mapping ecosystem services.

Researcher John A. Harrison of the School of the Environment at Washington State University also contributed to the study, titled “Reactive nitrogen inputs to US lands and waterways: how certain are we about sources and fluxes?” It was published in the March 2013 issue of Frontiers in Ecology and the Environment.